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Metamorphism Definition “The mineralogical, chemical, and structural adjustment of solid rocks to physical and chemical conditions which differ from the conditions under which the rocks in question originated” (from the Glossary of geology, 2nd edition) The term metamorphism refers to, “The mineralogical, chemical, and structural adjustment of solid rocks to physical and chemical conditions which differ from the conditions under which the rocks in question originated. (Glossary of geology, 2nd edition) It comes from a Greek word, meta morph, meaning change of form. In biology it is used to denote events like the change of a caterpillar to a butterfly. The changes in petrology occur between those of weathering, cementation, and diagenesis and melting. Diagenesis refers to all the processes that a sediment undergoes during lithification in near-surface environments. But the processes occurring during metamorphism have many similarities to those that occur during diagenesis. Thus, metamorphic changes are usually defined in a negative sense - all those not included in zones of weathering or diagenesis. This does not always lead to a clear separation. Notable problems: Zeolites - these open framework silicates often are generated in low-temperature diagenetic environments. They are also the product of environments in which recrystallization has clearly occurred. Alkali feldspars - These minerals are essentially high-temperature, but can be produced by the action of some liquids in diagenetic environments. Protolith - The term refers to the starting material from which a reaction or recrystallization begins from. Amorphous materials such as glass, or volcanic ash, organic matter, and evaporite minerals react much faster and at lower temperatures than most silicate or carbonate minerals. Since some standards must be established, the IUGS formed a Subcommission on the Systematics of Metamorphic Rocks (SCMR). The SCMR, a branch of the IUGS Commission on the Systematics in Petrology (CSP), was initiated in 1985. The Subcommission actually comprises 33 members, 11 Study Groups, and a Working Group of around 100 earth scientists spread worldwide. The work of this subcommission is still in progress. Metamorphism is a lengthy process and so, apparently, is the work of the subcommission. Where does metamorphism begin? In Hyndman’s original book, he stated the minimum temperature at 300̊C, occasionally lower. Winter suggests a temperature range of 100-150̊C for unstable protoliths. This is more in line with what is seen in rocks. A better limit might be above the boiling point of water under ambient conditions.
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Introduction to Metamorphism GLY 4310 - Spring, 2016
Petrology Lecture 9 Introduction to Metamorphism GLY Spring, 2016
Chemical systems attempt to equilibrate with the conditions they
are currently under. A system at high temperature is often at
equilibrium. If conditions change, the system will attempt to
change to match the new conditions. Solid rock generated at depth
may be brought toward the surface by orogenic or diapiric uplift,
accompanied by erosion. Changes occur in the uplifted rock. If
gabbro is exposed at the surface, mafic minerals weather to yield
oxides, serpentine, etc. and the feldspars alter to become clays.
The conditions under which the gabbro was generated and the
weathering conditions at the surface represent opposite extremes,
separated by 1000C and similar large differences in pressure,
oxygen fugacity, and presence of water. In between the extremes
lies the realm of metamorphism. Metamorphism Definition
The mineralogical, chemical, and structural adjustment of solid
rocks to physical and chemical conditions which differ from the
conditions under which the rocks in question originated (from the
Glossary of geology, 2nd edition) The term metamorphism refers to,
The mineralogical, chemical, and structural adjustment of solid
rocks to physical and chemical conditions which differ from the
conditions under which the rocks in question originated. (Glossary
of geology, 2nd edition) It comes from a Greek word, meta morph,
meaning change of form. In biology it is used to denote events like
the change of a caterpillar to a butterfly. The changes in
petrology occur between those of weathering, cementation, and
diagenesis and melting. Diagenesis refers to all the processes that
a sediment undergoes during lithification in near-surface
environments. But the processes occurring during metamorphism have
many similarities to those that occur during diagenesis. Thus,
metamorphic changes are usually defined in a negative sense - all
those not included in zones of weathering or diagenesis. This does
not always lead to a clear separation. Notable problems: Zeolites -
these open framework silicates often are generated in
low-temperature diagenetic environments. They are also the product
of environments in which recrystallization has clearly occurred.
Alkali feldspars - These minerals are essentially high-temperature,
but can be produced by the action of some liquids in diagenetic
environments. Protolith - The term refers to the starting material
from which a reaction or recrystallization begins from. Amorphous
materials such as glass, or volcanic ash, organic matter, and
evaporite minerals react much faster and at lower temperatures than
most silicate or carbonate minerals. Since some standards must be
established, the IUGS formed a Subcommission on the Systematics of
Metamorphic Rocks (SCMR). The SCMR, a branch of the IUGS Commission
on the Systematics in Petrology (CSP), was initiated in The
Subcommission actually comprises 33 members, 11 Study Groups, and a
Working Group of around 100 earth scientists spread worldwide. The
work of this subcommission is still in progress. Metamorphism is a
lengthy process and so, apparently, is the work of the
subcommission. Where does metamorphism begin? In Hyndmans original
book, he stated the minimum temperature at 300C, occasionally
lower. Winter suggests a temperature range of C for unstable
protoliths. This is more in line with what is seen in rocks. A
better limit might be above the boiling point of water under
ambient conditions. Onset of Metamorphism Minerals used to
characterize the onset of metamorphism include: Analcime,
carpholite, glaucophane, heulandite, laumontite, lawsonite,
paragonite, prehnite, pumpellyite, and stilpnomelane Minerals used
to characterize the onset of metamorphism include: Analcime,
carpholite, glaucophane, heulandite, laumontite, lawsonite,
paragonite, prehnite, pumpellyite, and stilpnomelane. The high
temperature boundary also has problems. We have seen that granitic
melts may begin to melt as low as 600C, but that melting is limited
by low volatile content, and may extend over hundreds of degrees.
The melt itself is igneous, but what is the residuum? When it is
the majority, it seems appropriate to call it metamorphic. What
about when the residuum is a few percent of the original rock?
Natural boundaries are vague, and petrologists must be prepared to
deal with these vague boundaries. Labels such as igneous or
metamorphic petrologist often necessarily blend together. Pressure
limits are also somewhat vague. Under low pressure conditions,
metamorphism may require quite high temperatures, seen only in
regions of very high geothermal gradients. Such gradients may exist
near the contacts with igneous intrusions, but seldom elsewhere.
Truly high-pressure rocks from the earths lower mantle or inner
core might be considered metamorphic, but they never reach the
surface, and we therefore dont deal with them. Kimberlite xenoliths
indicate pressures of >4 GPa (>120Km), and are considered
metamorphic. Mantle samples are also found in ophiolite sequences.
With these exceptions, most rocks are crustal. The upper limit for
pressure is thus about 3 GPa, usually lower. With these ideas in
mind, the SCMR has proposed the following definition of
metamorphism: SCMR Definition of Metamorphism
The Subcommission on the Systematics of Metamorphic Rocks has
proposed the following definition: Metamorphism is a subsolidus
process leading to changes in mineralogy and/or texture (for
example grain size) and often in chemical composition in a rock.
These changes are due to physical and/or chemical conditions that
differ from those normally occurring at the surface of planets and
in zones of concentration and diagenesis below the surface. They
may coexist with partial melting. Generally the study of coal,
petroleum, and ore deposits are excluded from petrology.
Anthracite, the highest grade of coal, is regarded as metamorphic.
Many ore formation processes involve metamorphic reactions. The
exclusion is based solely on the perceived need to have specialists
in these subjects. Agents of Metamorphism Any change in the
physical or chemical state of a rock can potentially lead to
metamorphism. Several agents, or types of change are commonly
recognized. These may act independently, but are often observed to
act in combination. Oceanic and Continental Geotherms
Figure 1.9. Estimated ranges of oceanic and continental
steady-state geotherms to a depth of 100 km using upper and lower
limits based on heat flows measured near the surface. After Sclater
et al. (1980), Earth. Rev. Geophys. Space Sci., 18, Temperature
Metamorphism Introduction, slide 5 here Changes in temperature
commonly contribute to metamorphic changes.Figure 1-9 shows the
ordinary oceanic and continental geotherms. The boundary lines
shown, between kms, represent mid to lower continental crust. The
continental geotherm is considerably higher, due to LIL elements,
which concentrate in continental crust and generate significant
heat. Increased heat promotes recrystallization, especially: 1.
When the rocks are fine-grained, because these have much higher
area/volume ratios, and reactions occur on surfaces. 2. When the
environment is static. Shear stresses typically reduce grain sizes.
The process of converting smaller grains to fewer, larger grains of
the same type is annealing, often observed in metals and ceramics
when these materials are held at high temperatures. Increased heat
may also take a mineral beyond its natural limit of stability. This
results in a reaction or reactions that consume the mineral,
forming a new stable phase. Kinetic barriers can and do impede this
process, sometimes precluding it from occurring until very much
higher temperatures are reached. Some substances, known as
mineralizers (effectively catalysts) reduce the kinetic barriers.
Fluoride ion is a very effective mineralizer, and water and carbon
dioxide may play the same role, The types of reactions which may
occur include: 1. Devolatilization reactions - This may include
dehydration or decarbonation, as water or carbon dioxide molecules
break bonds at increasing temperatures. Volatile-bearing minerals
must be in the original assemblage for these reactions to occur.
The higher the volatile content, the more likely these reactions
are to occur. Minerals such as clays, zeolites, chlorites, and
serpentines form at low temperature, and there very volatile rich
character makes them targets for diagenetic and low-temperature
metamorphic reactions. 2. Crystallization reactions - These include
the formation of new minerals, as well as the recrystallization of
existing minerals. The term neocrystallization is sometimes used to
specify that new minerals are bing formed. 3. Kinetic barrier - As
temperatures are raised, kinetic barriers that may preclude some
reactions form occurring may be overcome. These barriers prevent
the attainment of equilibrium at lower temperatures, but elevated
temperatures allow the rocks to equilibrate. Pressure Since most
rocks are metamorphosed at regions within the earth where
temperatures are higher, pressure has also necessarily increased.
Such pressure is variously known as lithostatic, confining, or load
pressure. Most rocks will find themselves under conditions
represented by the geothermal gradients curves of figure 1-9.
Perturbations of the geothermal gradient can and do occur: Lower
than average - Within a subducting plate Higher than average -
Above a hot spot, near an igneous intrusion, unusual radioactive
concentrations, near zones of crustal thickening or extension Much
perturbations are transient phenomena, and the effects will
diminish as the perturbations disappears. Examination of old
orogenic belts can yield useful information about the pressure
temperature relationships that have occurred under different
conditions. Metamorphic Trajectories
Figure 21-1 shows a plot of data from studies that have been done.
Typically these data are generated by traversing a metamorphic
outcrop from the lowest toward the highest grade of metamorphism.
Highest grade conditions are usually at the greatest depths.
However, the traverses rarely represent a true vertical
cross-section, and are thus not representative of the exact
geothermal gradient. Post-metamorphic uplift and erosion are
uneven, accounting for many of the problems observed. The
metamorphic field gradients, aka metamorphic trajectories or
arrays, give us the best estimate we have of the true geothermal
gradient at the time of metamorphism. Actual trajectories may vary
across an orogen, increasing toward the point where heat and
plutonism were highest. Minerals have stability limits for pressure
as well as temperature. Increasing pressure may cause the
disappearance of a mineral, replacing it with a new phase. The
different trajectories seen in Figure 21-1 may thus show quite
different replacement patterns. For example, the Franciscan path is
nearly vertical, indicating rapid change in pressure with little
increase in temperature. The various trajectories for Scottish
rocks show a much higher rate of temperature change, often with
modest pressure increases. The term metamorphic grade is often used
to express the degree of metamorphism. Generally more attention is
paid to temperature than pressure in determining metamorphic grade.
If we accept the role of pressure as being subsidiary to
temperature, than pressure can be a modifier. When pressures are
low, less dense phases will form, whereas high pressures favor
denser minerals. Figure Metamorphic field gradients (estimated P-T
conditions along surface traverses directly up metamorphic grade)
for several metamorphic areas. After Turner (1981). Metamorphic
Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill.
Pressure Types Lithostatic pressure - uniform stress
(hydrostatic)
Deviatoric stress = pressure unequal in differentdirections
Resolved into three mutually perpendicular stress(s) components: s1
is the maximum principal stress s2 is an intermediate principal
stress s3 is the minimum principal stress In hydrostatic situations
all three are equal (Roll mouse) The type of pressure is also
important. Lithostatic pressure is equal in all directions.
Tectonic pressures are usually directed along an axis. When this
occurs, ductile flow usually occurs in a way that reduces the
effect of the imposed pressure. (Le Chtlier again) This happens
whenever the imposed pressure differential exceeds the strength of
the rock. This happens below a relatively shallow depth. Above this
depth, the rocks behave in a brittle manner, fracturing and then
moving along the fracture (= faulting). Deviatoric Stress (roll
mouse) Deformation, common in metamorphic rocks, is seen only when
pressure is unequal in different directions. Lithostatic pressure,
which is uniform (equal in all directions), can produce only equal
contraction, not deformation. Deviatoric stress (unequal in
different directions) is produced by tectonic forces.Three
dimensional vectors (such as force) can be resolved into three
principal components. The stress tensor can be resolved into three
principal components, as follows: (roll mouse - 3 times) 1 is the
maximum principal stress 2 is the intermediate principal stress 3
is the minimum principal stress (roll mouse) For uniform stress, 1
= 2 = 3. Stress will be maintained as long as differential pressure
is applied, and keeps pace with the rocks ability to yield.
Examples include orogenic belts, extending rifts, and shear zones.
The yielding by the rock is deformation, aka strain. Stress is
force/unit area, and the deformational response is strain.
Deviatoric Stress Effect
Deviatoric stress affects the textures and structures, but not the
equilibrium mineralassemblage Strain energy may overcome kinetic
barriers to reactions Deviatoric Stress: Tension
There are three types of stress: Tension - Forces pull an object
apart, and the resulting strain is extension. 1 and 2 positive, but
3 is negative (See Figure 21-2a). This often results in tension
fractures. These fractures allow metamorphic fluids to circulate,
and may be filled by minerals precipitated from solution. Figure
The three main types of deviatoric stress with an example of
possible resulting structures. a. Tension, in which one stress in
negative. Tension fractures may open normal to the extension
direction and become filled with mineral precipitates. Winter
(2001) An Introduction to Igneous and Metamorphic Petrology.
Prentice Hall. Deviatoric Stress: Compression
Compression - Forces directed along a single axis toward a common
center. 1 is dominant, with little contribution from 2 or 3. The
response to compression may be folding, which shortens the object
in the direction of applied stress (2 3), or flattening (2 3) (see
Figure 21-2b). Platy or elongated minerals may be rotated during
either folding or flattening. Figure The three main types of
deviatoric stress with an example of possible resulting structures.
b. Compression, causing flattening or folding. Winter (2001) An
Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Flattening s1 > s2 = s3 foliation and no lineation
s1 = s2 > s3 lineation and no foliation s1 > s2 > s3 both
foliation and lineation Figure 21-3b shows the flattening of a
sphere into an ellipsoid (actually, the strain ellipsoid) Rigid
minerals have rotated to be perpendicular to the applied stress. We
can easily observe the orientation of minerals after deformation,
but have no means of judging the shape deformation, since we dont
know the original shape. Thus, observation of foliation is
extremely important. Working with oriented samples, we can estimate
the directions of the principle stress components. With large scale
deformation, field evidence will usually have already given us this
information. For small scale deformation, the petrologic
examinations may provide crucial input. New minerals may also grow
during deformation, and will tend to grow 1 . Whether by rotation
or flattening, This leads to metamorphic foliation, which may be
schistosity, gneissosity, or rock cleavage. Compression in two
directions simultaneously is also possible. In this case, 1 2 >
3 and the resulting strain ellipsoid will be elongated, but roughly
circular in cross-section perpendicular to the direction of
elongation (think of a salami). Elongated and platy minerals will
rotate so that their long axis is parallel to the axis of
elongation. This produces lineation. Another possibility is that 1
> 2 > 3. In this case, the strain ellipsoid is elongated in
two directions, one greater than the other, rather like a bar of
soap. Both foliation (of platy minerals) and lineation (of
elongated minerals) will occur. The platy minerals will be
perpendicular to the 1 and 2 axis. The elongated minerals will be
perpendicular to 1. Another response to stress of the compressive
type is shear. Shear results from movement along a plane or set of
planes at an angle to 1. This may produce a stain ellipsoid
identical to that produced by flattening. In structural geology,
flattening is referred to as pure shear (Figure 21-3b), Figure
Flattening of a ductile homogeneous sphere (a) containing randomly
oriented flat disks or flakes. In (b), the matrix flows with
progressive flattening, and the flakes are rotated toward
parallelism normal to the predominant stress.Winter (2001) An
Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Deviatoric Stress: Shear
whereas the movement along planes is called simple shear (Figure
21-2c). Pure shear results in foliation to the short axis of
deformation, while in simple shear the foliation is not orthogonal
to the shortest axis of deformation. Metamorphic Fluids Many
metamorphic reactions release fluids, and their may have been a
fluid phase associated with the parent rock. Fluid refers to either
liquid or gas. Indeed, at low pressures the fluid may be either
liquid, gas, or both. For water, above the critical point, the
liquid and gas phases merge into a single fluid phase. Figure The
three main types of deviatoric stress with an example of possible
resulting structures. b. Shear, causing slip along parallel planes
and rotation. Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall. Phase Diagram for Water
Fig After Bridgman (1911) Proc. Amer. Acad. Arts and Sci., 5, ;
(1936) J. Chem. Phys., 3, ; (1937) J. Chem. Phys., 5, (See Figure
6-7) Since many metamorphic reactions occur at conditions above the
critical point (374C, 21.8 MPa in pure water), they may only be a
single phase present, called a super-critical fluid. If
electrolytes or other dissolved fluids (CO2) are present in the
water, the position of the critical point is elevated to higher
temperatures and pressures. Direct evidence for the existence and
composition of the fluid phase is difficult to obtain. As the rocks
are brought to the surface, fluids escape. Sometimes there may be
fluid inclusions within mineral grains. Careful study of the fluids
will give some indication of composition, but under much lower T
and P conditions than the original fluids were at. If the fluid
inclusions occur as planar arrays, chances are they are
post-metamorphic fluids they have entered the rock along cracks,
and then be trapped. Theoretical evidence for fluid phases comes
from the mineral assemblages themselves, since many would not be
stable in the absence of any fluids. At shallow depths, the fluid
phase extends continuously to the surface. Lithostatic pressures
will be = gh. The fluids, in contact with the surface, will also
experience a pressure ofgh. But the two are not equal. For
lithostatic pressure = mineral. For water, = water. Since water
< mineral, Pfluid < Plith. The presence of impervious
sedimentary caprock may increase Pfluid, but it is still less than
Plith. Collapse Below a certain depth, around 10 km, the pressure
at the point of mineral contact, Plith, will be very much greater
then the pressure exerted by the intergranular fluids on the
minerals (see Figure 21-4). One of two things happens: 1. The
mineral grains deform, and compress the pore space until Pfluid =
Plith. 2. Pressure Solution occurs. Minerals at the stressed
contacts between grains have a higher free energy than adjacent
grains not in contact. They may dissolve and be reprecipitated in
the pore space. This lowers the overall free energy of the system,
and also allows the grains to move closer together, as well as
filling the pore spaces. This continues until Pfluid = Plith. Most
intergranular fluids are water. Carbon dioxide may also be present,
and methane is sometimes present, depending on whether the system
is oxidizing or reducing. Other possible species include S gases
and N2. Dissolved species include alkalis and halides. The presence
of multi component fluid phases may be handled on the basis of
pressure, Figure A situation in which lithostatic pressure (Plith)
exerted by the mineral grains is greater than the intergranular
fluid pressure (Pfluid). At a depth around 10 km (or T around
300oC) minerals begin to yield or dissolve at the contact points
and shift toward or precipitate in the fluid-filled areas, allowing
the rock to compress. The decreased volume of the pore spaces will
raise Pfluid until it equals Plith. Winter (2001) An Introduction
to Igneous and Metamorphic Petrology. Prentice Hall.
Multi-Component Fluid Phases
Pfluid = PH2O + PCO XH2O + XCO = 1.0 PH2O = XH2O Pfluid Pfluid =
PH2O + PCO or from a mole fraction viewpoint, where the sum of the
mole fractions total 1, XH2O + XCO = 1.0 The two are related: PH2O
= XH2O Pfluid There are a variety of potential fluid sources. These
include: Potential Fluid Sources
1. Meteoritic water 2. Juvenile Water 3. Water associated with
subducted material 4. Sedimentary brines 5. Water from metamorphic
dehydration reactions 6. Degassing of the mantle Metamorphic fluids
are hot and under pressure. They are capable of considerable
dissolution, followed by transport. A similar situation exists with
waters released from plutonic intrusions into sedimentary country
rock. These fluids may exchange substances with the material
through which they pass. This process is known as metasomatism.
Analysis of many metamorphic rocks shows that most metamorphic
reactions are essentially isochemical (no addition or removal of
atoms). However, limitations on measurements of fluid phase
movements mean we ignore the introduction or removal of atoms by
fluid phases. Since metasomatism involves considerable exchange,
there is a gap between the terms isochemical and metasomatic, with
no adequate terminology to describe intermediate stages.
Metamorphic reactions often involve gradients in T and P. To that,
we should add metasomatic (chemical) gradients. IUGS-SCMR
Classification of Metamorphic Rocks
Contact Metamorphism Pyrometamorphism Regional Metamorphism
Orogenic Metamorphism Burial Metamorphism Ocean Floor Metamorphism
Hydrothermal Metamorphism Fault-Zone Metamorphism Impact or Shock
Metamorphism Types of Metamorphism The IUGS-SCMR has recommended
the following classification for metamorphic rocks. (above) Contact
Metamorphism Agents: Heat from igneous intrusion, possible
metasomatic intrusion Location: Anywhere an intrusion occurs, over
a wide range of depth (pressure) - effects are greatest in shallow
intrusions because country rock is much colder than the intrusion
Contact Metamorphism A contact aureole typically surrounds the
intrusion. Epizonic plutons have relatively narrow aureoles,
because heat is rapidly lost into the cold country rock. Mesozonic
aureoles are wider, but the country rocks are likely to be
metamorphic, and the effects are harder to observe. In catazonic
plutons, the country rocks are already so hot that contact
metamorphic effects are often insignificant. Heat from the lava
flow above baked sediments Into a red shale in a narrow zone along
the contact Temperature Distributions Within a Dike
Figure Temperature distribution within a 1-km thick vertical dike
and in the country rocks (initially at 0C) as a function of time.
Curves are labeled in years. The model assumes an initial intrusion
temperature of 1200C and cooling by conduction only. After Jaeger,
(1968) Cooling and solidification of igneous rocks. In H. H. Hess
and A. Poldervaart (eds.), Basalts, vol. 2. John Wiley & Sons.
New York, pp Figure 21-5 shows a model by J.C. Jaeger on the
thermal effects of a dike emplaced at 1200C into country rocks at
0C. One kilometer from the intrusion, temperatures are raised 200C,
but only after several thousand years. Once raised to this
temperature, it maintains the temperature for a million years or
more. The exact effects depend on the volume of the intrusion, the
shape of the intrusion, the orientation of the intruding body, and
the initial difference in temperature. Batholiths may have contact
aureoles stretching for several kilometers, but for small dikes the
effects reach only centimeters or even millimeters. If the country
rock is permeable and fluids are available, fluids will cool the
intrusion quicker, and will heat the country rock faster and at a
greater distance from the contact. Metasomatism plays the greatest
role when the composition of the country rock is substantially
different from that of the magma, especially when the country rock
is carbonate-rich. The hot acidic waters react with carbonates to
produce rocks known as skarns or tactites. The fluid source, as
determined by oxygen isotope data, is usually meteoritic, but some
may be juvenile. In shallow, low-pressure environments, the effects
of contact metamorphism are most evident. There is typically low to
very low deviatoric stress, and the rocks are not foliated. A
typical texture is that of hornfels or granofels. Relict structures
are preserved. Hornfels Photo: Hornfels - A fine-grained granofels,
characteristic of contact metamorphic aureoles. Granofels - A
texture characterized by a lack of preferred orientation, aka
isotropic texture. Many times plutonism accompanies orogeny.
Contact metamorphic effects occur in conjunction with orogenic
metamorphism. The contact metamorphism overprints the regional
metamorphism, especially in shallow, low-grade environments.
Spotted slates or phyllites are an example of this. The slate or
phyllite is produced during orogenic metamorphism. Later, some
grains of the rock begin to grow as the result of reheating by the
contact event. Pyrometamorphism is a very high temperature type of
contact metamorphism. Xenoliths which fall into magma chambers may
be rapidly heated, and show substantial alteration. Wall rocks in
or near volcanic necks are another example. Partial melting of
country rock is common. Regional Metamorphism Any metamorphic event
that effects a large body of rock and has a lateral extent of tens
of kilometers may be considered to be regional. We may separate
these rocks into three subcategories, Orogenic Metamorphism Agents:
Dynamo-thermal - Deviatoric stress produced by plate collision,
along with elevated geothermal gradients. Location: Island arcs,
continental arcs, and continental collision zones all fall into
this category. Convergent plate margins exert substantial
deviatoric stress on rocks at the boundary. Many of the studies of
regional metamorphism focus on orogenic belts, so the terms
orogenic and regional are sometimes regarded as synonymous.
Affected rocks are normally foliated, ranging from slate through
gneiss. Pinkish upper layer is Shap Granite, which intruded as hot
magma into the surrounding rocks around 400 million years ago. Heat
from the magma intrusion has baked the darker rock (which was
originally mudstone), causing it to re-crystallize in a new form a
hard, flinty-looking metamorphic rock called hornfels. Continental
Arc Orogen
Figure Schematic model for the sequential (a c) development of a
Cordilleran-type or active continental margin orogen. The dashed
and black layers on the right represent the basaltic and gabbroic
layers of the oceanic crust. From Dewey and Bird (1970) J. Geophys.
Res., 75, ; and Miyashiro et al. (1979) Orogeny. John Wiley &
Sons. An example of a continental-arc orogen is shown in Figure The
incipient induction is shown is figure 21-6a. Beneath the
developing trench, tectonic pressures will be quite high, resulting
in high-pressure metamorphism. Part b shows the development of an
orogenic welt, caused by crumpling within the continental plate.
The crust will thicken due to compression, tectonic underplating,
and magmatic underplating. Tectonic underplating refers to a
trench-ward migration of the underthrusting by the oceanic plate,
leaving accumulating slabs of oceanic crust on the welt.Magmatic
underplating occurs when mantle-derived melts stall and accumulate
at the base of the crust. This adds heat to the welt from below by
magmatic underplating, rising plutons, and induced mantle
convection above the subducted slab. In addition, the thickened
crust generates more radioactive heat. Temperature is greatest at
the center of the welt, and increases downward toward the plutons.
Metamorphism is extensive. Partial melting may occur. The resulting
structures are described as gneiss domes or even metamorphic core
complexes. The Adirondack Mountains in New York state are a classic
example. Uplift of the welt and the inevitable erosion exposes the
metamorphic rocks. The rocks remain hot for a considerable period,
and metamorphism continues. This tends to make the metamorphic
pattern a simple concentric dome, although the structural pattern
that accompanies the metamorphism may be complex. Many orogenic
episodes produce repeated episodes of deformation and metamorphism,
again leaving a polymetamorphic imprint. Collision between two
continents involves a plate with a passive margin. Passive
continental margins are usually the site of massive sediment
accumulation (continental shelf). The structural, magmatic, and
metamorphic patterns are even more complex than for a
continent-ocean collision. Burial Metamorphism Agents: Load
pressure, temperature due to geothermal gradient Location:
Sedimentary basins In sedimentary basins, sediments may accumulate
to thicknesses of ten kilometers or more. The conditions near the
bottom of the pile may be just enough to produce low-grade
metamorphism. The Southland Syncline in New Zealand is one example.
The sediments were mainly volcanoclastics, fine glassy material
that is very prone to reaction. Diagenesis reactions will occur in
the higher levels. At somewhat lower levels, very mild metamorphism
in the form of new minerals like zeolites, prehnite, pumpellyite,
and laumontite ere found. At the deepest levels, minerals typical
of greenschist facies are found. Most of the metamorphism is
restricted to veins, strongly suggesting that fluids were involved.
Opal is also found. This suggests the fluids are silica
saturated.The hydrothermal fluids may indicate some elevation of
the geothermal gradient. Coombs, who proposed the term burial
metamorphism, also proposed calling this type of metamorphism
hydrothermal metamorphism. Since some degree of hydrothermal
alteration is present in many rocks, this is a difficult type of
metamorphism to define, and the term is only partially accepted. A
major difference between orogenic metamorphism and burial
metamorphism is the lack of significant structural deformation in
the latter. The sedimentary basins are large, nearly undisturbed
accumulations of sediment. Modern examples include the Gulf of
Mexico, fed principally by the Mississippi River, and the Bengal
Fan, fed by the Ganges and Brahmaputra Rivers. Seismic studies
indicate the Bengal Fan is 22 kilometers deep. This would
correspond to a pressure of 0.6 GPa, and temperatures between C.
This is well within minimum metamorphic conditions. Some foliation
due to load pressure is possible. Rocks from the bottom of such a
basin might be very difficult to distinguish from low grade
orogenic rocks. Ocean-floor metamorphism Agents: Metasomatism,
temperature Location: Ocean crust near MOR spreading centers In the
past few decades, post WWII ocean exploration has recovered a
considerable amount of metamorphosed gabbro and basalt from the
ocean floor near spreading centers. Miyashiro et al. called the
process that created these rocks ocean-floor metamorphism. The
minerals observed in these rocks indicate a wide variety of
temperatures, all at low pressure. There is considerable chemical
alteration, especially the replacement of Ca and Si by Mg and Na.
This type of alteration is typical of basalt reacting with
seawater. There are very substantial localized differences in the
degree of metamorphism, almost certainly due to proximity to
fractures that allow seawater to circulate. Direct evidence comes
from submersible observation of black smokers, and from drill
cores. Fractures zones range from major, spaced kilometers apart,
through meter-sized openings down to centimeter sized fractures
attributed to contraction upon cooling. Alteration happens quickly,
almost always near the ridge, where magmatism and heat are
greatest. Indeed, the localization of this metamorphism to near
ridge environments caused Spear to suggestthe term Ocean-ridge
metamorphism. It is clearly a type of hydrothermal alteration.
Actual alteration involves a process called albitization of the
plagioclase found in basalt. Sodium from seawater replaces Ca.
Alteration of feldspars and mafics also produces chlorite, calcite,
epidote, prehnite, zeolites, and other low-temperature minerals.
The resulting product is called a spillite. It often retains the
textures of the basalt. These include both vesicles and pillow
structures. Of particular note are the distinct chlorite-quartz
rocks. They have a high-Mg, low-Ca signature. No other igneous or
sedimentary rock has this signature. They may be the protolith of
the cordierite-anthophyllite metamorphic rocks, found under higher
grade conditions in some orogenic belts. Fault-Zone Metamorphism
Agents: Shear stress Location: Along the fault scarps associated
with large scale faulting, and in distributed shear zones This
process goes under several names. SCMR sometimes calls the process
dislocation metamorphism, and shear-zone metamorphism is often
used. Spear has used the term high-stress metamorphism and dynamic
metamorphism is used. The latter two terms include both fault-zone
and impact metamorphism. The metamorphism seen here is the result
of strained crystal lattices. Strain weakens bonds, and greatly
lowers energy barriers that often inhibit reactions. If
temperatures are low, and strain high, the response is bending or
breaking, sometimes crushing of grains. This process is called
cataclasis, and occurs in the shallow portions of fault zones where
minerals are brittle. Rocks in the brittle zone are fault breccia
(broken, crushed filling in fault zones) or fault gouge (clayey
alteration of breccia by groundwater permeating down the fault
plane). Mylonite Photo: At greater depths, localized shearing may
produce a fine-grained foliated rock called a mylonite. Mylonite
along the Linville Falls Fault, Linville Falls, NC. Relatively
undeformed conglomeratic quartzite lies above the layered mylonite
zone. (Text from Pamela Gore) Fault Zone Cross-Sections
(a) Shallow fault zone with fault breccia (b) Slightly deeper fault
zone (exposed by erosion) with some ductile flow and fault mylonite
(See figure 21-7) At larger depths, shear movements are more evenly
distributed because rocks are ductile. This requires higher
temperatures, and recrystallization begins to accompany the shear.
It is also important to remember that different types of minerals
become ductile at different conditions. Quartz may still be quite
brittle while calcite might be ductile. Impact Metamorphism Agent:
Transient, extremely high pressure wave, accompanied by rapid very
large temperature increase Location: At the point of impact of
large meteorite impact Rocks formed when a large bolide impacts the
earth at speeds about 10 km/sec sustain a transient but very large
pressure wave. In addition, temperature is quickly raised several
thousand degrees. This results in some melting, and sometimes
vaporization, of minerals. These rocks, called impactites, are
foundin the impact crater and associated ejecta blanket. Impactites
often possess one or more features. High pressure silica phases,
either coesite or stishovite, may be present.Amorphous glass is
usually present. Figure Schematic cross section across fault zones.
After Mason (1978) Petrology of the Metamorphic Rocks. George Allen
& Unwin. London. Shocked Quartz Geology cover and text: Figure:
Shocked quartz crystals, containing characteristic shock lamellae,
may be present. Shocked quartz crystal from the K/T boundary layer
of the Raton Basin, Colorado/New Mexico. Photo by Glen Isett, US
Geological Survey Shatter Cones Figure Shatter cones in limestone
from the Haughton Structure, Northwest Territories Photograph
courtesy Richard Grieve, Natural Resources Canada Shatter cones,
which are nested cone-shaped structures are also characteristic.
Prograde Metamorphism
Prograde: increase in metamorphic grade with timeas a rock is
subjected to gradually more severeconditions Prograde metamorphism:
changes in a rock thataccompany increasing metamorphic grade
Retrograde: decreasing grade as rock cools andrecovers from a
metamorphic or igneous event Retrograde metamorphism: any
accompanyingchanges Progressive Metamorphism (Roll mouse wheel )
Metamorphism may accompany any change in conditions. These could
include an increase in temperature or pressure. The process is
called prograde, (roll mouse wheel) and the resulting changes in
the rock are prograde metamorphism.(Roll mouse wheel ) A decrease
in temperature and pressure is called retrograde, (roll mouse
wheel) and resulting changes are retrograde metamorphism. In all
except extreme cases, such as incipient metamorphism, or transient
events like fault-zone or impact metamorphism, equilibrium is
thought to be obtained, and to prevail during an on-going
metamorphic episode. The Progressive Nature of Metamorphism
A rock at a high metamorphic grade probablyprogressed through a
sequence of mineralassemblages rather than hopping directly from
anunmetamorphosed rock to the metamorphic rockthat we find today
Metamorphic rocks usually maintain equilibrium as grade increases
High-grade metamorphic rock probably progressed through a sequence
of mineralassemblages as it adjusted to increasing temperature and
pressure, rather thanhopping directly from un-met to the
metamorphic rock that we find today If a metamorphosed sedimentary
rock experienced a cycle of increasingmetamorphic grade, followed
by decreasing grade, at what point on this cyclic P-T-tpath did its
present mineral assemblage last equilibrate? The zonal distribution
of metamorphic rock types preserved in a geographicsequence of
increased metamorphic grade suggests that each rock preserves
theconditions of the maximum metamorphic grade (temperature)
experienced by thatrock during metamorphism The Progressive Nature
of Metamorphism
Retrograde metamorphism typically of minorsignificance Prograde
reactions are endothermic and easilydriven by increasing T
Devolatilization reactions are easier thanreintroducing the
volatiles Geothermometry indicates that the mineralcompositions
commonly preserve the maximumtemperature Retrograde is usually
detectable by observing textures, such as the incipient replacement
ofhigh-grade minerals by low-grade ones at their rims Zonal
metamorphic patterns, in which rocks preserve a geographic sequence
of increasing grade, suggests that retrograde metamorphism is not
terribly significant. Since cooling in large metamorphic complexes
should be slow, time should not be a significant factor in
precluding retrograde reactions. What might inhibit them? (Roll
mouse wheel) 1. Prograde reactions are endothermic. The heat
supplied causes them to progress rapidly. They often involve
dehydration or decarbonation, or both. This helps to maintain
equilibrium. 2. Retrograde reactions are exothermic. However, this
assumes the fluids (water or carbon dioxide) are available for
rehydration or recarbonation reactions. In most cases, the fluids
are driven off during prograde reactions, and are not available.
Thus, the mineralogy and texture of metamorphic rocks usually
reflects the maximum grade obtained. The composition may not,
however. Geothermobarometry uses the temperature and sometimes the
pressure dependence of metamorphic reactions between coexisting
minerals to estimate the T and P conditions of metamorphism.
Exchange reactions, such as Mg-Fe, do not require much energy. They
maintain equilibrium with the retrograde conditions, at least until
temperatures fall to the point where the reactions become extremely
sluggish. Protolith Types The parent rock type has an extremely
important effect on the types and degree of metamorphism. The most
important factor is the composition of the parent rock. Six
categories serve to separate the parent rocks. Types of Protolith
Lump the common types of sedimentary and igneousrocks into six
chemically based-groups 1.Ultramafic - very high Mg, Fe, Ni, Cr
2.Mafic - high Fe, Mg, and Ca 3.Shales (pelitic) - high Al, K, Si
4.Carbonates - high Ca, Mg, CO2 5.Quartz - nearly pure SiO2.
6.Quartzo-feldspathic - high Si, Na, K, Al Chemistry of the
protolith is the most important clue toward deducing the parent
rock (Roll mouse wheel for each number) 1.Ultramafic rocks. Mantle
rocks, komatiites, or cumulates 2.Mafic rocks. Basalts or gabbros,
some graywackes 3.Shales (or pelitic rocks). Fine grained clastic
clays andsilts deposited in stable platforms or offshore wedges.
4.Carbonates. Mostly sedimentary limestones and dolostones.Impure
carbonates (marls) may contain sand or shale components 5.Quartz
rocks. Cherts are oceanic, and sands are moderately highenergy
continental clastics. Nearly pure SiO2. 6.Quartzo-feldspathic
rocks. Arkose or granitoid and rhyoliticrocks. High Si, Na, K, Al
Categories are often gradational, and cannot include the full range
of possible parental rocks One common gradational rock type is a
sand-shale mixture: psammite Other rocks: evaporites, ironstones,
manganese sediments, phosphates, laterites, alkaline igneousrocks,
coal, and ore bodies Regional Metamorphism, Scottish
Highlands
Barrows Area Figure Regional metamorphic map of the Scottish
Highlands, showing the zones of minerals that develop with
increasing metamorphic grade From Gillen (1982) Metamorphic
Geology. An Introduction to Tectonic and Metamorphic Processes.
George Allen & Unwin. London. Orogenic Regional Metamorphism,
Scottish Highlands George Barrow, British petrologist, made one of
the first systematic studies of metamorphic rock types, their
variation, and mineral assemblages in an episode of progressive
metamorphism. His studies were centered in the SE Highlands of
Scotland, starting in the late 19th century. These rocks were
produced by the Caledonain orogeny, which peaked about 500 mybp.
Deformation was intense. The rocks were folded and thrust into a
series of nappes, with numerous large granitic intrusions. Later
plate movements fragmented the orogenic belt, and pieces of it are
found in Scandinavia, Greenland, and in the Appalachian belt of N.
America. The Scottish rocks comprise a total thickness of 13
kilometers, composed of conglomerate, sandstone, shale, limestone,
and mafic rocks. Barrow found little change in the sandstones. The
pelitic rocks (protolith: shale) he divided into a series of
metamorphic zones based on the appearance of a new mineral in each
zone. He also observed that grain size increased through the zones.
The minerals used to denote the zones are called index minerals.
The zones now recognized in the Scottish highlands are: (See Figure
21-8) Barrovian Zones Biotite zone - Phyllites, schists
Chlorite zone - Slates or phyllites Minerals: Chlorite, muscovite,
quartz, albite Biotite zone - Phyllites, schists Minerals: Biotite,
chlorite, muscovite, quartz, albite Garnet zone: Garniferous
schists Minerals: Red almandine garnet, biotite, chlorite,
muscovite, quartz, albite or oligoclase Staurolite zone: Schists
Minerals: Staurolite, biotite, muscovite, quartz, garnet and
plagioclase Kyanite zone - Schists Minerals: Kyanite, biotite,
muscovite, quartz, and plagioclase, garnet, staurolite. Sillimanite
zone - Schists and gneisses Minerals: Sillimanite, biotite,
muscovite, quartz, plagioclase, andgarnet, staurolite, kyanite
(Roll mouse wheel to show zones successively) This sequence has
been recognized in a number of orogenic belts. The zones are now
well-established as Barrovian zones. C.E. Tilley added the term
isograd for the boundary that separates zones. The classical
definition of isograd is a line connecting points of the first
appearance of a particular metamorphic index mineral in the field
as one progresses up the metamorphic grade. When an isograd is
crossed, a new zone is entered. The zone has the same name as the
isograd which marks its lower boundary. An isograd is intended to
represent a line of constant metamorphic grade. It is, of course,
difficult to find continuous outcrops in the field. Thus the
boundaries may be somewhat fuzzy in areas of poor exposure. In
addition, isograds are probably irregular curved surfaces in space,
and the mapped isograds represent the intersection of the
three-dimensional isograd with the earth's surface. An index
mineral may be present in zones higher than its own. Even when the
mineral should have been replaced, it may persist. The Al2SiO5
polymorphs serve as examples. Al2SiO5 Phase Diagram (See Figure
21-9) The kyanite to sillimanite reaction involves small changes in
free energy, so the kyanite may persist metastably. (It is possible
that conditions are exactly on the phase boundary, but the
appearance of two minerals in the same zone happens much more
commonly than it should by chance position in a phase diagram.)
Barrovian zones were developed in an area of rather narrow
compositional range. In regions with different compositions, the
use of additional or replacement index minerals may be appropriate.
In the Appalachians, the shales are more iron and aluminum rich
than in the Scottish Highlands. It is possible to introduce a
chloritiod isograd in such cases. In the Banff and Buchan districts
of Scotland, the isograds are as follows: Buchan or Abukuma
Isograds
Chlorite Biotite Cordierite Andalusite Sillimanite The appearance
of andalusite instead of kyanite is an indication that pressures
were low, since andalusite is not stable above 0.37 GPa. Cordierite
has a large molar volume, a good indication that it too is a low
pressure mineral. Thus it appears the geothermal gradient was
higher in the Buchan area, and pressures were lower at equivalent
temperatures. This sequence is known as Buchan-type metamorphism.
In Japan, the same variation is known as the Abukuma-type. Regional
Burial Metamorphism, Otago, New Zealand From the Permian through
the Jurassic, much of the island of New Zealand was the location of
voluminous sedimentation. This included graywacke, tuff and some
volcanics, deposited in a trough. This combination of rocks has a
very fine grain size making them very prone to metamorphism. During
the Cretaceous, these rocks were metamorphosed. Regional Burial
Metamorphism Otago, New Zealand
Section X-Y shows more detail (See Figure 21-10) Studies by D.S.
Coombs and coworkers on the South Island of New Zealand have made
this the "type locality" for burial metamorphism. In the northern
part of the island, medium grade Barrovian zones are reached. But
in the south, we see lower grade rocks that are another variation.
There is little deformation here, and plutons are almost
non-existent. Figure Geologic sketch map of the South Island of New
Zealand showing the Mesozoic metamorphic rocks east of the older
Tasman Belt and the Alpine Fault. The Torlese Group is
metamorphosed predominantly in the prehnite-pumpellyite zone, and
the Otago Schist in higher grade zones. X-Y is the Haast River
Section of Figure From Turner (1981) Metamorphic Petrology:
Mineralogical, Field, and Tectonic Aspects. McGraw-Hill. Haast
Group Figure Metamorphic zones of the Haast Group (along section
X-Y in Figure 21-10). After Cooper and Lovering (1970) Contrib.
Mineral. Petrol., 27, The isograds the Coombs group mapped here are
shown in Figure 21-11, which is of the Haast River section. The
isograds are: Zeolite Prehnite-pumpellyite Pumpellyite(-actinolite)
Chlorite (-clinozoisite) Biotite Almandine garnet Oligoclase The
zeolite zone is characterized by the alteration of volcanic glass
to the zeolites heulandite or analcite, accompanied by secondary
quartz and some phyllosilicates. The original minerals are
essentially unaltered, even high temperature volcanics which are
far from their field of stability. As depth increases, heulandite
is replaced by laumonite, and both prehnite and pumpellyite form.
Plagioclase is albitized because the most calcic compositions are
unstable. As grade increases, the rocks are more recrystallized,
and good schistosity develops. The rocks then evolve into grades
associated with regional orogenic metamorphism. At very low grade,
reaction rates are slow. Equilibrium is probably not obtainable in
geologically reasonable times. The preservation of original igneous
textures and minerals is common. Where hot fluids have penetrated,
grades are higher. Zonation is apparent if the rocks are carefully
observed, and the index minerals altered. The rocks in New Zealand
represent the various lowest levels of metamorphism, actually a
transition from diagenesis to metamorphism. We might expect to see
similar zones in the outermost areas of most orogenic belts, but
careful work has found them only in a few cases. It seems likely
the unstable tuffs and graywackes react faster than most rock. Hot
fluids have also increased reaction rates. Typical pelitic
sediments seem incapable of reaction under similar conditions.
Ca-bearing minerals, like laumonite, prehnite, and pumpellyite are
stable in water-rich, carbonate-free fluids. In the presence of
carbon dioxide, calcium would form calcite instead. In the Salton
Sea geothermal field of Southern California, hot fluids contain
carbon dioxide. Zeolites are absent, but epidote and chlorite are
common. Paired Orogenic Metamorphic Belts of Japan On the islands
of Shikoku and Honshu in Japan, a pair of metamorphic belts are
present along a NE-SW line parallel to the subduction zone. Both
belts are the same age, but they have different metamorphic
signatures. Japanese Metamorphic Belts
Figure The Sanbagawa and Ryoke metamorphic belts of Japan\ From
Turner (1981) Metamorphic Petrology: Mineralogical, Field, and
Tectonic Aspects. McGraw-Hill and Miyashiro (1994) Metamorphic
Petrology. Oxford University Press. (See Figure 21-12) The Abukuma
or Ryoke belt, which lies to the NW, farthest away from the
subduction zone, is a Buchan type low P/T belt. Rocks are
meta-pelitic sediments ranging up to sillimanite zone. Granitic
plutons are common. These rocks represent high-temperature,
low-pressure conditions. The Sanbagawa belt, which lies closer to
the subduction zone, is composed of late Paleozoic
volcanic/sedimentary fill, with grade increasing toward the NW. It
shows much higher pressures than the Ryoke belt. The highest grade
reached is garnet, in pelitic rocks. Alkaline rocks are more
common, however, and glaucophane develops in the alkaline rocks,
giving way to hornblende at higher grade. Glaucophane is an
important indicator mineral, denoting high-pressure low temperature
conditions. The bluish color of the glaucophane is used in naming
the rocks blueschists. The two belts are entirely separated by a
major fault zone called the Median Line. The Ryoke-Abukuma belt is
derived from sediments typical of a mature volcanic arc. The
Sanbagawa belt is formed from the oceanward accretionary wedge,
with a mixture of arc-derived sediments and volcanics mixed
together with oceanic crust and marine sediments. It is the marine
sediments that contribute the high sodium content necessary to form
glaucophane. Subduction Zone Isotherms
Figure shows a cross-section of a subduction zone. The 600C
isotherm is as deep as 100 km in the trench-subduction zone area,
whereas it can be as shallow as 20 km below the volcanic arc. Thus
the Ryoke-Sanbagawa belts probably represent a pair of coeval
belts. A higher P/T belt will be near the trench, and a lower P/T
belt away from the trench. These should be common in many
ocean-continent or ocean-ocean subduction zones. These were called
Paired Metamorphic Belts by Miyashiro. Figure Cross section of
subduction zone showing isotherms (after Furukawa, 1993) and mantle
flow lines (dashed and arrows, after Tatsumi and Eggins, 1995).
Potential magma source regions are numbered. Circum-Pacific
Metamorphism
Figure Some of the paired metamorphic belts in the circum-Pacific
region From Miyashiro (1994) Metamorphic Petrology. Oxford
University Press. Figure shows a map of the circum-Pacific belt,
showing five examples, and one more in the Caribbean. The paired
belts are separated by kms, an area called the arc-trench gap. The
contact between the belts in the ocean-continent case is often a
major fault.The offset along the faults is dip-slip, but
considerable strike-slip component may be present. Contact
Metamorphism of Pelitic Rocks, Skiddaw Aureole, UK In the English
Lake District, Ordovician Skiddaw Slates are intruded by granite
and granodiorite bodies. The intrusions are shallow. Contact
metamorphism overprints earlier low-grade regional orogenic
metamorphism. The orogenic metamorphism reached chlorite-zone.
Outside of the contact aureole, the slates contain muscovite (or
sericite-phengite, at the lowest grades), quartz, chlorite,
chloritoid, and mixed opaques (graphite, sulfides, mixed
Fe-oxides), biotite. They have been divided into three zones,
mainly on the basis of texture. Contact Metamorphism of Pelitic
Rocks in the Skiddaw Aureole, UK
Unaltered slates Outer zone of spotted slates Middle zone of
andalusite slates Inner zone of hornfels Skiddaw granite Increasing
Metamorphic Grade The width of the aureole, 2 km, suggests that the
exposed igneous contact dips outward at a small angle. Skiddaw
Granite, Lake District, UK
Figure Geologic Map and cross-section of the area around the
Skiddaw granite, Lake District, UK After Eastwood (1et al. 968).
Geology of the Country around Cockermouth and Caldbeck Explanation
accompanying the 1-inch Geological Sheet 23, New Series. Institute
of Geological Sciences. London. The outer zone is characterized by
small spots ( mm). The finer material has recrystallized, and is
slightly coarser than matrix in the unaltered slate. Thee spots are
black because the opaques seem to concentrate around the center of
crystallization. Thin-section examination of the spots and matrix
reveals essentially the same mineralogy, except for slightly more
muscovite in the spots. The spots were probably a single grain
which surrounded nearby minerals, and were likely either andalusite
or cordierite. Retrograde metamorphism has reduced them to a fine
aggregate, dominated by muscovite. Both cordierite and andalusite
are found at higher grades, including the inner-most part of the
outer zone, where they are corroded. This scenario makes the spots
pseudomorphs. The middle zone contains biotite, muscovite,
cordierite, andalusite, quartz, and opaque minerals. Cordierite
forms irregular, equi-dimensional crystals with numerous
inclusions. Mica inclusions retain the orientation of the external
slate matrix, indicating cordierite enveloped aligned micas. This
is evidence for the overprint of thermal metamorphism on earlier
regional metamorphism. Skiddaw Aureole, UK Middle Zone
Slates more thoroughlyrecrystallized Contain biotite +muscovite +
cordierite +andalusite + quartz 1 mm Figure Cordierite-andalusite
slate from the middle zone of the Skiddaw aureole From Mason (1978)
Petrology of the Metamorphic Rocks. George Allen & Unwin.
London. Andalusite crystals contain fewer inclusions than
cordierite, and may have cruciform inclusions of opaque minerals
(chiastolite - Figure shows incipient andalusite, figure a
chiastolite grain). Skiddaw Aureole, UK Inner Zone
Thoroughly recrystallized Foliation lost 1 mm Figure
Andalusite-cordierite schist from the inner zone of the Skiddaw
aureole Note the chiastolite cross in andalusite (see also Figure
22-49). From Mason (1978) Petrology of the Metamorphic Rocks George
Allen & Unwin. London. Both andalusite and cordierite are
low-pressure minerals, so must represent the contact metamorphism
episode, which occurred at shallow depths. The inner zone minerals
are coarser and more recrystallized than the middle zone, but have
the same mineral assembly. Cordierite has no mica inclusions, but
contains trains of opaques which may record the original foliation
direction. Some rocks are schistose, but the innermost zone loses
all foliation, and the rocks are hornfelsic.The quartz-rich areas
shown in figure meet in triple junctions, at about 120, typical of
nearly hydrostatic metamorphism. The zones at Skiddaw are
structural, rather than on a grade basis. The sequence of mineral
development with grade is difficult to determine. Most minerals
appear in the outer zone. Thus the textural approach appears more
useful at Skiddaw. This may extend to other contact metamorphic
aureoles as well. In Scotland, Comrie schists were intruded by much
hotter (diorite) rocks than at Skiddaw. The inner aureole is
granofels, containing very high temperature mineral associations
(opx + K-spar). These minerals form by dehydration of biotite and
muscovite. Opx, in both contact and regional metamorphic rocks,
appears only at the highest grades, usually accompanied by partial
melting. Typical mineral assemblage: Comrie Schists, Scotland
Typical Mineral Assemblage: Hypersthene + cordierite + orthoclase +
biotite + opaques Silica rich rocks Cummingtonite (Ca-free
amphibole) + quartz + andesine + biotite + opaques Silica
undersaturated Corundum, Fe-Mg spinel Contact metamorphosed by
diorite intrusion, much hotter than the Skiddaw Intrusion Since the
silica undersaturated regions occur only in the inner aureole,
Tilley attributed their presence to Si loss to the diorite. More
likely, SiO2 and water entered partial melts, and were removed.
Contact Metamorphism and Skarn Formation, Crestmore, California The
Crestmore Quarry in Los Angeles California is a world famous
location for contact pyrometamorphism involving metasomatism. The
quarry is operated by a cement company, which uses to products for
the manufacture of cement. Most of the aureole is now gone.
Burnham's 1959 study provides an excellent view of this classic
location. Contact Metamorphism, Crestmore, California
Figure Idealized N-S cross section (not to scale) through the
quartz monzonite and the aureole at Crestmore, CA From Burnham
(1959) Geol. Soc. Amer. Bull., 70, A quartz monzonite porphyry was
intruded into Mg-bearing carbonates. The country rock had
experienced an earlier metamorphic event, and were brucite-bearing
calcitic marbles. The temperatures involved at Crestmore were high
enough to make this a case of pyrometamorphism. Considering the
shallow depth (as evidenced by low pressure minerals) this is very
unusual, seen in only a few places on earth. Burnham's study mapped
four zones, consisting of a total of 10 assemblages. Crestmore
Zones Mineral Formulas Brucite Mg(OH)2 Calcite CaCO3
ClinohumiteMg9(SiO4)4(F,OH)2 ClinoniteCa(Mg,Al)32Al2Si2O10(OH)2
DiopsideCaMgSi2O6 ForsteriteMg2SiO4 Grossular (garnet)Ca2Al2Si3O12
Melilite(Ca,Na)2(Mg,Al)(Si,Al)2O7 MerwiniteCa3Mg(SiO4)2
MonticelliteCaMgSiO4 SpinelMgAl2O4 SpurriteCa5(SiO4)2(CO3)
TillyiteCa5Si2O7(CO3)2 Vesuvianite
(Idocrase)Ca19(Al,Mg,)13B0-5Si18O68(OH,O,F)10 WollastoniteCaSiO3
This kind of representation represents information overload. It is
difficult to remember all the assemblages, and the chemistry that
is taking please is difficult to interpret.It is clear there are a
progression of minerals, such as the replacement of clinohumite by
clintonite between assemblages 2 and 3. The appearance of the index
minerals is also clear. In order to help clarify what is actually
happening, we can rewrite the information in the form of chemical
equations Example Transformations
Assemblage 1 to Assemblage 2 2 Clinohumite + SiO2 9 Forsterite + 2
H2O Assemblage 7 to Assemblage 8 Monticellite + 2 Spurrite + 3
Merwinite + 4 Melilite + 15 SiO H2O 6 Vesuvianite + 2 CO2 The
chemical equations provide additional information, but also have
draw-backs. They aren't easy to determine. They also are difficult
to remember. But they do provide information not clear from the
mineral assemblages alone, such as the liberation of carbon dioxide
during the formation of vesuvianite.This has lead to a new trend,
treating all isograds as chemical reaction, whenever possible.
Large quantities of numerical data are often handled best by
displaying the data graphically.Examination of the mineral list
reveals that most of the minerals can be represented by the system
CaO-MgO-SiO2-CO2-H2O. Omitting the volatiles, the system can be
plotted on the ternary diagram CaO-MgO-SiO2 (Figure 21-18).
CaO-MgO-SiO2 Zones are numbered (from outside inward)
Figure CaO-MgO-SiO2 diagram at a fixed pressure and temperature
showing the compositional relationships among the minerals and
zones at Crestmore. Numbers correspond to zones listed in the text.
After Burnham (1959) Geol. Soc. Amer. Bull., 70, ; and Best (1982)
Igneous and Metamorphic Petrology. W. H. Freeman. CaO-MgO-SiO2
Zones are numbered (from outside inward) Such diagrams are called
chemographic compatibility, or composition-paragenesis diagrams.
Figure shows the diagram for Crestmore. All of the minerals in the
assemblages are plotted on the diagram. Coexisting minerals are
connected by lines, called "tie-lines". The numbers represent the
mineral assemblages discussed previously. Any point which lies
within a small triangle created by the tie-lines will consist of
the minerals at the corners of the triangle. Points lying on lines
will consist of just the two minerals at opposite ends of the line.
Presented in this manner, a trend leaps off the page. The mineral
assemblages become increasing rich in silica. Since Mg-bearing
carbonates have little silica available, the only likely source of
silica is from silica-rich fluids released from the magma. The
porphyry nature of the intrusive rock supports this idea. When the
magma is rising, crystals form and grow, becoming phenocrysts. When
the magma gets close to the surface, the volatiles may escape. This
quickly undercools the magma (raises the melting point) and the
remaining magma solidifies in tiny crystals. The escaping
volatiles, especially water, are hot and loaded with dissolved
silica. The heat from the escaping water adds to convective heat
loss, and creates a pyrometamorphic situation. The silica reacts
with the carbonates, creating the assemblages seen. Alumina and
volatiles were omitted. Care must be chosen in selecting the three
end members. Computers have actually aided the process, since they
can plot all possible diagrams for examination. This removes a lot
of the fun of discovery, however, since no intuition is
involved.